1. Field of the Invention
The present invention relates to an apparatus for charging a lithium battery while adjusting the ambient temperature of the lithium battery pack.
2. Description of Related Art
As mobile apparatuses are advancing, more and more large-capacity secondary batteries, such as nickel-cadmium batteries or nickel-hydrogen batteries, are used as power supplies in portable CD players with a radio receiver, portable video cameras, portable personal computers, communications devices such as cellular telephones, and power apparatuses such as cordless electric tools. Lithium battery that is one of the large-capacity secondary batteries is attracting attention as one for use in portable apparatuses, because the lithium battery can provide more electric power than the other types of secondary batteries.
The nominal voltage of the lithium battery is about two to three times greater than that of the nickel-cadmium battery or nickel-hydrogen battery. The energy density of the lithium battery is about three times higher than that of the nickel-cadmium battery. In addition, the lithium battery is small and light. Further, the discharge efficiency is high. The battery can discharge even in a relatively low-temperature environment, and can provide a constant voltage over a broad temperature range.
Taking these charactering features of the lithium battery into account, the inventors hereof have studied the use of the lithium battery pack as motor driving power supply in cordless electric tools such as impact drivers, so that the cordless electric tools may become as small and light as possible. It is preferable to make cordless electric tools, in particular, lighter as much as possible because workers keep holding the tools for a long time at the site where the tools are used. In view of the above circumstances, use of lithium batteries for the cordless electric tools is advantageous. Unlike other electronic apparatuses, cordless electric tools use a large motor. Therefore, use of the lithium battery, which has a large capacity (i.e., large discharge capacity), as a driving source is advantageous.
Chargers designed to charge battery pack incorporated in cordless electric tools are frequently brought to the working site such as construction site, together with cordless electric tools. If a cordless electric tool is used for a long time, the power of the battery pack used in the tool will decrease. As a result, the battery pack needs to be charged by the charger. Further, if the charger is used at a site in a cold region, the battery pack cannot be charged well. In view of this, the charger should have a function (rapid-charging function) of charging the battery pack as fast as possible and should charge the battery pack in conditions, where the charging-discharging function of the battery pack is not impaired.
Conventionally, chargers that have a function of controlling the environmental temperature (ambient temperature) of secondary batteries are well known. Japanese Patent Application Publication No. HEI 7-87673 discloses a charger that sets the ambient temperature of a secondary battery at a preset value before charging the secondary battery so that the discharge capacity of the secondary battery is not impaired.
Charging the battery at too low temperature will cause the electrolyte generating unnecessary gas. Charging the battery at too high temperature will shorten the lifetime of the battery. Therefore, Japanese Patent Application Publication No. HEI 8-185897 discloses a charger that sets the lower temperature limit to a value equal to or higher than a preset value, and sets the upper temperature limit to a value equal to or lower than a preset value.
Further, Japanese Patent Application Publication No. HEI 8-223815 discloses a charger that adjusts the ambient temperature of a secondary battery (e.g., lead storage battery) using a heating unit to prevent a decrease in the capacity of the secondary battery. Japanese Patent Application Publication No. HEI 11-55869 discloses a charger that adjusts the ambient temperature of a secondary battery within a predetermined range (0° C. to 40° C.) to prevent the battery performance from being degraded. Japanese Patent Application Publication No. HEI 11-288744 discloses a charger that charges a secondary battery at a predetermined temperature while controlling the temperatures of the plurality of batteries constituting a battery pack to become a same value.
Japanese Patent Application Publication No. 2001-155783 discloses a lithium-battery charger for use in artificial satellites, which prevents a non-aqueous electrolyte battery from freezing at low temperatures and prevents the battery from deteriorating at high temperatures, thereby to impart a stable charge-discharge characteristic to the battery.
The lithium secondary battery is different from the nickel-hydrogen secondary battery in nature. Therefore, the lithium battery must be charged by a method different from the above-described method of charging the nickel-hydrogen secondary battery, to maintain the charge-discharge characteristic or the long lifetime. If the lithium secondary battery is just charged at a constant current within a predetermined temperature range by the conventional method, the time of rapid charging cannot be shortened or the lifetime (charge-discharge cycle lifetime) of the lithium battery cannot be lengthened.
Further, the temperature of self-heating during the charging of the lithium secondary battery is low. If the lithium battery is charged at an ambient temperature lower than any temperature falling within the optimal charging-temperature range (e.g., 15° C. to 35° C.), the lithium battery must be charged at a low current, in consideration of the characteristic. The conventional charging method does not work only if the time of charging is shortened.